1. Field of the Invention
The present invention relates to a technique analyzing impurity distribution and surface shape of a semiconductor specimen.
2. Discussion of the Background
As one of methods analyzing impurity distribution, there is a SCM (Scanning Capacitance Microscopy) measurement that is disclosed to "J. of Elec. Mat. Vol25, No2, p301, 1996".
FIG. 34 is a block diagram showing overall configuration of a conventional SCM measuring apparatus 1. The SCM measuring apparatus 1 in FIG. 34 comprises a stage 11 on which a specimen 16 is placed, XY scanning circuit 12 that scans the stage 11 in the XY direction, a control circuit 13 that controls XY scanning circuit 12, data storing section 14 in which measuring data, control data and the like are stored, a probe 17, a tip 15 of which is contacted on surface of the specimen 16, a sensor 18, and CV measuring apparatus 19.
The signal detected by the probe tip 15 is inputted to the sensor 18 via a cantilever 20, and then amplified. After that, the amplified signal is inputted to the CV measuring apparatus 19 via a UHF transfer line L1.
The SCM measuring apparatus 1 measures the capacitance by the same principle as that of a UHF resonance capacitance sensor. When the probe tip 15 is put on the specimen 16, all of the probe tip 15, the sensor 18, the transfer line L1, and the specimen 16 constitute a part of the resonator. That is, the fluctuation of the capacitance C between the probe tip 15 and the specimen 16 is equivalent to the load, and due to the load, the resonance frequency changes. As a result, with a little change of the resonance frequency, the resonance amplitude changes considerably. By means of this resonator, the sensitivity of attofarads (10.sup.-18 F) is obtained.
The SCM measuring apparatus 1 in FIG. 34 gives the desired change of the capacitance in the specimen 16 adjacent to the probe tip 15, by supplying the electric field (AC bias of kHz band-width) between the probe tip 15 and the specimen 16.
Free carriers beneath the probe tip 15 is induced or repelled to the probe tip 15 in order to form depletion state or accumulation state. Such depletion state and accumulation state are equivalent to the case changing the distance between the capacitors.
The depth of the depletion layer, that is, the change of the distance between the plates of the capacitor is determined by three factor, that is, i) intensity of supplied electric field; ii) quality and thickness of a dielectric between the probe tip and the measuring object, iii) concentration of the free carriers.
It is assumed that the carrier shields and terminates the supplied electric field. The more intensive the electric field is, or the lower the concentration of the carrier is, the depletion layer is formed until the location deep from the surface. Conversely, the weaker the electric field is, or the higher the concentration of the carrier is, the depletion electric field ends nearby of the surface.
For the specimen that has both of the region with high carrier concentration and the region with low carrier concentration, in case of comparing by the supply voltage with the same level, the region with low carrier concentration has thicker depletion layer.
The SCM measuring apparatus 1 in FIG. 34 measures the moving of the carriers. The lower the carrier concentration is, or the thinner the surface oxidation layer is, the signal with higher signal intensity is outputted. The signal obtained by the SCM measurement is a dC/dV, that is, the change of the capacitance of the depletion layer for the change of the supplied voltage. In the SCM measurement, because the alternating voltage is supplied on the surface of the specimen, the above-mentioned dV may be considered the peak-to-peak voltage. In other words, the above-mentioned dV may be considered a changing amount of all the depletion layer formed beneath the probe tip.
The SCM measuring apparatus 1 of FIG. 34 outputs the relationship between the voltage V supplied on the surface of the specimen and the capacitance C by the form of C-V curve. More specifically, the SCM measuring apparatus 1 converts a modulated component dC of the capacitance in case of supplying a constant voltage amplitude dV to the specimen into a image. Further, the DC bias for the specimen is also capable of adjusting. By adjusting the DC bias, the standard voltage of AC bias changes.
FIG. 35 is a figure showing typical high-frequency CV property of n-type semiconductor. In case of p-type semiconductor, the polarity of the CV property is contrary to FIG. 35. As shown in FIG. 35, when a positive bias voltage is applied to the gate terminal or the probe tip, inversion electrons are induced on the surface of the semiconductor. In the strong inversion state, the total capacitance of the capacitor is equal to that of the dielectric which is usually a oxidation layer.
On the other hands, in case of changing the voltage supplying to the probe tip in the negative direction, the depletion layer enlarges and the capacitance goes down. Further, as the lower the concentration of the carrier is, the more early the depletion layer enlarges, and the capacitance goes down quickly when the voltage changes. That is, the SCM measuring apparatus can regard as a gradient measuring apparatus of the CV property.
By the way, as one method analyzing the surface of the specimen, an AFM (Atomic Force Microscopy) is being known. FIG. 36 is a block diagram showing overall configuration of an AFM measuring apparatus 5. The AFM measuring apparatus 5 in FIG. 36 comprises a probe 21, a piezoelectric element (PZT) 23 on which the specimen 22 is placed, a XY scanning circuit 24 that scans the PZT 23 in the XY direction, a control circuit 25 that controls the XY scanning circuit 24, a data storing apparatus 26 in which measuring data, control data and so on are stored, a servo circuit 27 that controls the PZT 23, a photo-detector 28, a mirror 29, and laser diode 30.
When the distance between the probe tip 31 and the specimen 22 changes within the range of 1 .mu.m-100 .ANG., the following force works between the probe tip 31 and the specimen 22. Within the distance close to the surface of the specimen, i.e. about 100 .ANG., an inter-atomic force works mainly. Within the distance of about 3-4 .ANG. from the surface of the specimen, a repulsive force works mainly. Over more than the distance, an attractive force works mainly. On the other hands, at the far distance from the surface of the specimen, an static electricity force due to an electric dipole of the electric charge or the polarity material.
The AFM measuring apparatus 5 in FIG. 36 changes asperity on the surface of the specimen into displaced amount of a cantilever 32, and then detects the displacement amount by using a principle of the optical lever. More specifically, the laser diode 30 rays out the laser for the probe tip 31, and the photodetector 28 detects the reflecting light from the probe tip 31. The servo circuit 27 moves the specimen 22 put on the PZT 23 in up and down direction so that the reflective light aggregates to the center of the photo-detector 28, and the signal moving the PZT 23 in up and down direction is changed into image showing the surface shape of the specimen.
The above-mentioned SCM measuring apparatus 1 in FIG. 34 is capable of analyzing the impurity distribution inside the specimen. The above-mentioned AFM measuring apparatus 5 in FIG. 36 is capable of analyzing the surface shape of the specimen. However, in case of performing the SCM measurement and AFM measurement by using the above-mentioned conventional apparatuses, there are the following problems.
Firstly, because the probe tip has width with about several hundred .ANG., it is virtually impossible to measure the width smaller than several hundred .ANG.. That is, in case of performing the SCM measurement or the AFM measurement, the resolution is restricted by the width of the probe tip. Further, in case of performing the SCM measurement, the carrier density at the end of the depletion layer inside the specimen is detected by assuming the capacitor plates located in parallel. However, practically, because the shape of the probe tip does not have enough width being able to assume as the plate, the deviation occurs in case of assuming the end of the depletion layer in the specimen as the parallel plate.